Xringer,
I have a similar unit. They prevent overcharging, prevent backloss of battery current to PV at night, cut out and prevent load operation when battery voltage gets too low.
Three LEDs show state of battery charge (two of the three are on), another LED lights up when PV panel is charging battery and a third LED indicates when a load is present.
Here is the link to the e-bay site describing this controller. You can get them in 5, 10, 20 and 30A configurations. I got the 5 A model.
Solar Panel Battery Charge Controller 30A Amp 12V 24V Solar PV System Black | eBay
Steve